Input and return values of stats_cdf_t depends on the $which-parameter:
<?php
function get_t_p($t, $df){
return stats_cdf_t($t, $df, 1);
}
function get_t_t($p, $df){
return stats_cdf_t($p, $df, 2);
}
function get_t_df($p, $t){
return stats_cdf_t($p, $t, 3);
}
?>